EP1045 - From IVF to Imatinib: Diagnosis of a Rare Distal Rectal GIST

Purpose/Background: Gastrointestinal stromal tumors (GIST) represent a relatively commonly encountered intestinal malignancy. Most GISTs arise from the proximal intestinal tract, with stomach and small intestine representing the two most common origins. Based on population-based studies, anorectal GISTs account for only 4% of all reported GISTs. Given the rarity of anorectal GISTs, case reports drive clinical understanding of this disease. This report describes the clinical course of a patient who presented with a pelvic mass during pregnancy, which was identified after delivery of her child as a GIST. Her diagnostic course was made more complicated by her wanted pregnancy at time of discovery of this mass, and treatment plans tailored to her postpartum desire to breastfeed and high-risk lesion in a difficult anatomic location.

Methods/Interventions: The patient is a forty-four-year-old female who underwent in-vitro fertilization to achieve a successful pregnancy. During a routine 25-week fetal ultrasound, a well-circumscribed, echogenic pelvic mass was identified inferior and posterior to the cervix, measuring 31 x 25 x 30 mm. The patient denied any clinical symptoms. An MRI of the pelvis obtained one month later demonstrated a 3.8cm well circumscribed and solid mass, arising from the superior anal canal versus distal rectum (Figure 1). Imaging review was concerning for a neoplastic process, with differential including lymphoma. Further evaluation was delayed until after delivery. However, the patient developed spontaneous preterm delivery at 30 weeks and underwent an emergent cesarian section. After an uneventful delivery, the patient was referred to colorectal surgery for further evaluation of her pelvic mass. After discussion at the institutional multi-disciplinary tumor board, recommendation was made to proceed with excisional biopsy.

Results/Outcomes: The patient underwent a rectal examination under anesthesia, and digital rectal examination revealed a palpable, firm but mobile mass in the distal rectum in the left anterior location, without involvement of the anal sphincter. This was confirmed to be submucosal on both the endoscopic and digital exams. Vaginal exam revealed no involvement with the vaginal mucosa. A mucosal incision was made directly over the mass and a well encapsulated mass was manually grasped and delivered through the mucosal incision. Pathology demonstrated a gastrointestinal stromal tumor, spindle cell type, with piecemeal resection of a 3.5cm mass (with 19 mitoses per 5 mm², G2, high grade).

Conclusion/Discussion: The National Comprehensive Cancer Network (NCCN) guidelines define high risk GIST based on mitotic index as a mitotic count greater than 5 per 5 mm². For this reason, our patient was referred to medical oncology with plans to start therapy with Imatinib. This will require the patient to terminate breast feeding in her early postpartum state to avoid potential adverse effects to her infant. Given that surgical pathology was unable to confirm negative margins, as well as the high-grade lesion and high-risk features, a full total mesorectal excision would be offered in the form of an ultra-low anterior resection or abdominoperineal resection. Given the rarity of a large, low rectal GIST, case reports such as this drive further understanding of this disease process. This case represents a unique clinical challenge given the patient's ongoing pregnancy at time of discovery and offers potential guidance for other providers on how to balance a desire to maintain a healthy pregnancy with acceptable oncologic outcomes.

Figure 1
MRI pelvis. Pelvic mass (A) and Fetal brain (B)